Many known absorbent composites that include a high-absorbency material include the high-absorbency material in relatively low concentrations. That is, many of the absorbent composites include airlaid cellulosic fibers and less than about 30 weight percent of a high-absorbency material. This is due to several factors.
Many high-absorbency materials are unable to absorb a liquid at a rate at which the liquid is applied to the absorbent composites during use. Accordingly, a relatively high concentration of fibrous material is desirable to temporarily hold the liquid until the high-absorbency material can absorb it. Further, the fibers serve to separate the particles of high-absorbency material so that gel-blocking does not occur. Gel-blocking refers to the situation wherein particles of high-absorbency material deform during swelling and block the interstitial spaces between the particles, or between the particles and the fibers, thus preventing the flow of liquid through the interstitial spaces.
U.S. Pat. No. 5,147,343 issued Sep. 15, 1992, to Kellenberger describes an absorbent composite adapted to avoid the problem of gel-blocking. U.S. Pat. No. 5,147,343 describes the use of a superabsorbent material which can absorb at least 27 milliliters of a 0.9 weight percent aqueous sodium chloride solution per gram of superabsorbent material while the superabsorbent is under a restraining pressure of at least 21,000 dynes per square centimeter. When the superabsorbent material is in the form of discrete particles, at least about 50 percent by weight of the superabsorbent material has a size greater than the median pore size of the porous fiber matrix when wet. The described absorbent composites are said to contain up to about 90 weight percent of a superabsorbent material.
The presence of a relatively low concentration of high-absorbency material and a relatively greater concentration of fibrous materials has resulted in the production of absorbent composites which tend to be relatively thick. In some instances, the use of a relatively thick absorbent composite in a disposable absorbent garment is acceptable. However, in recent years it has become increasingly desirable to produce absorbent composites which are thin compared to the more traditional absorbent composites but which still possess the same absorbent capacity. Thin absorbents provide for a greater garment-like appearance as well as improved discretion when worn under other garments. The desire to produce relatively thin absorbent composites has resulted in the desire to incorporate ever-increasing amounts of high-absorbency material into the absorbent composites. This is because the absorbent capacity of such high-absorbency materials is generally many times greater than the absorbent capacity of fibrous materials. For example, a fibrous matrix of wood pulp fluff can absorb about 7-9 grams of a liquid, (such as 0.9 weight percent saline) per gram of wood pulp fluff, while the high-absorbency materials can absorb at least about 15, preferably at least about 20, and often at least about 25 grams of liquid, such as 0.9 weight percent saline, per gram of the high-absorbency material.
U.S. Pat. No. 5,601,542 issued Feb. 11, 1997, to Melius et al. describes an absorbent composite including a superabsorbent material contained by a containment means. The superabsorbent material has a Pressure Absorbency Index of at least 100 and a 16-hour extractables level of less than about 13 weight percent; a Pressure Absorbency Index of at least 100 and a Vortex Time of less than about 45 seconds; or a Pressure Absorbency Index of at least about 110. The superabsorbent material is present in the containment means in an amount of from about 30 to about 100 weight percent based on the total weight of the containment means and the superabsorbent material.
U.S. Pat. No. 5,149,335 issued Sep. 22, 1992, to Kellenberger et al. is directed to an absorbent structure containing a relatively high concentration of superabsorbent material. Specifically, U.S. Pat. No. 5,149,335 describes the use of a superabsorbent material having certain absorbent characteristics when it is desired to employ the superabsorbent material at relatively high concentrations. Specifically, the superabsorbent material is described as having a 5-minute Absorbency Under Load value of at least about 15 grams per gram and a free-swell rate of less than about 60 seconds.
In striving for thin absorbent composites, other desirable qualities are often sacrificed, such as capacity and flexibility. Quite often, when absorbent pads are densified to create high capacity in a thin form, hard spots develop within the pads, thereby resulting in stiffness and lack of uniformity of the absorbent material within the pads. On the other hand, when thin pads are made having a lower density, the resulting pads may be flexible, but thin, low density pads have a low absorbent capacity. Low density, high capacity pads that are flexible are generally thick and bulky and look and feel cumbersome on the wearer.
Various technologies are known for making absorbent pads. High capacity absorbent pads are typically produced on a conventional absorbent drum former by combining superabsorbent polymer and fluff pulp in a forming chamber. High superabsorbent polymer concentrations and uniform mixing of the absorbent components can lead to superabsorbent polymer containment issues during forming and in the finished product. Superabsorbent polymer loss during pad forming can cause high absorbent variability, inconsistent performance, raw material waste and process upset by loading the recycle system with superabsorbent polymer.
There is thus a need or desire for absorbent pads that are thin, flexible, and have a high absorbent capacity.
There is a further need or desire for a process for making well-mixed, uniform absorbent pads.
There is yet a further need or desire for a process for making absorbent pads in which superabsorbent polymer loss can be minimized.